The present invention generally relates to endoprosthesis devices, most often referred to as stents, and more particularly pertains to increasing the radiopacity of such devices.
Stents or expandable grafts are implanted in a variety of body lumens in an effort to maintain their patency and are especially well-suited for the treatment of atherosclerotic stenosis in blood vessels. Intercoronary stents have become a standard adjunct to percutaneous coronary angioplasty in the treatment of arterial atherosclerotic disease. Although commercial stents vary in design and materials, they share similar structural features. Most current stents in clinical use are metallic and are either self-expanding or are expanded by the force of an expandable member, such as an angioplasty dilatation balloon. These devices are typically implanted via a delivery catheter which is inserted at an easily accessible location on the patient and then advanced through the patient""s vasculature to the deployment site. The stent is initially maintained in a radially compressed or collapsed state to enable it to be maneuvered through the lumen and into the stenosis. Once in position, the stent is deployed which, depending upon its construction, is achieved either automatically by the removal of a restraint, or actively by the inflation of a balloon about which the stent is carried on the delivery catheter.
The stent must be able to simultaneously satisfy a number of mechanical requirements. First and foremost, the stent must be capable of withstanding the structural loads that are imposed thereon as it supports the lumen walls. In addition to having adequate radial strength or more accurately, hoop strength, the stent should nonetheless be longitudinally flexible to allow it to be maneuvered through a tortuous vascular path and to enable it to conform to a deployment site that may not be linear or may be subject to flexure. The material of which the stent is constructed must allow the stent to undergo expansion which typically requires substantial deformation of localized portions of the stent""s structure. Once expanded, the stent must maintain its size and shape throughout its service life despite the various forces that may come to bear upon it, including the cyclic loading induced by the pulsatile character of arterial blood flow. Finally, the stent must be biocompatible so as not to trigger any adverse vascular responses.
Fluoroscopy has typically been relied upon to facilitate the precise placement of a stent as well as to verify the position of a stent within a patient throughout its service life. The use of radiopaque materials in the construction of the stent allows for its direct visualization. Unfortunately, no single material to date has been identified that simultaneously satisfies all requirements inherent in a stent application. Those materials that do satisfy the mechanical requirements are either insufficiently or excessively radiopaque and/or have not been adequately proven to be biocompatible in a vascular setting. Thus, with current stent materials, constructing a radiopaque stent wholly out of a single material has not provided an optimal solution. A number of different approaches, however, have been employed wherein different materials are combined in an effort to render a mechanically sound and biocompatible stent to be visible by a fluoroscope system.
Several metals, such as stainless steel, nickel titanium alloys, tantalum and platinum alloys have been used to construct stents. These materials vary widely in their mechanical properties and radiopacity. All these materials can, by varying the design, be used to create the sent. However, the mechanical and radiopacity characteristics are not independent, but linked. Strength requirements dictate, for example, the strut thickness, geometry and percentage of the arterial wall which is to be covered by the stent structure. The resulting radiopacity is largely fixed and can only be adjusted with an alteration of the mechanical characteristics of the stent. For some materials, such as tantalum, the resulting stents can be too radiopaque, which results in obscured images of the anatomy in the stent lumen. This makes, for example, visualization of any possible restenosis within the stent very difficult to visualize on a fluoroscope. Other stent designs comprising of less radiopaque materials, such as stainless steel or nitinol, can have excellent mechanical functionability, but offer sub-optimal radiopacity except in cases where the stent struts can be very thick, as in an aortic stent-graft. In addition, the short-term hemocompatability and long term biocompatability of stents could be improved. In a short time-frame, the issue of stent thrombogenicity may be critical since modern coronary stents have a low, but measurable, rate of short term (one to seven days) thrombotic occlusion. This is true even if the patient is provided with systemic anticoagulation therapy. Metals such as tantalum and stainless steel, although inert, are actually coated with serum proteins and, to the extent that they are still activated, platelets after insertion into the bloodstream. In long-term implantation, stents become endothelialized. Therefore, biocompatability, particularly the foreign body response, can be of great concern. Growth of smooth muscle cells with extra cellular matrix production may lead to the restenotic closing of the arterial lumen. Platelet derived growth factor from thrombus-bound platelets can stimulate smooth cell muscle cells to proliferate. Metal ions that leech from the stent may catalytically oxidized low density lipo-proteins which exacerbate the original atherosclerotic condition.
One means frequently described for accomplishing fluoroscopic visibility is the physical attachment of radiopaque markers to the stent. Conventional radiopaque markers, however, have a number of limitations. Upon attachment to a stent, such markers may project from the surface of the stent, thereby comprising a departure from the ideal profile of the stent. Depending on their specific location, the marker may either project inwardly to disrupt blood flow or outwardly to somewhat traumatize the walls of the blood vessel. Additionally, galvanic corrosion that might result from the contact of two disparate metals, i.e., the metal used in the construction of the stent and the radiopaque metal of the marker could corrode, and in the worst case, cause the marker to become separated from the stent which could be problematic should the marker be swept downstream. Although such markers are typically fairly small, this approach does cause the radiopaque material to come into direct contact with living tissue which may be problematic should there be any biocompatibility issues. Finally, markers also give an incomplete picture of the stent expansion and orientation. Usually there are two markers, one at each end. By making the entire stent radiopaque and visible, its degree of expansion and curvature for its full length can be assessed.
Stents also have been previously marked by coating selected portions thereof with radiopaque material. Radiopaque metals, such as gold, platinum and tantalum can be coated by sputtering, evaporation or electroplating processes. It is important that these coated layers have good adhesion and conform to the stent during deformation. The deformation is typically greatest during stent expansion. However, a number of disadvantages are associated with this approach as well. This again causes the radiopaque material to come into direct contact with living tissue which, depending on the total area that is coated, can amount to a sizeable exposure. Unfortunately, cracking, flaking and delamination can be a problem with this approach. When the stent is expanded and certain portions thereof are caused to undergo substantial deformation, there is a risk that cracks would form in the plating and that sections thereof would become separated from the underlying substrate. This has the potential for causing turbulence in the blood flowing thereover to thereby induce thrombogenesis. Depending on the size and number of particles, pieces will create an embolized hazard for downstream vasculature. Moreover, once the underlying structural material becomes exposed, interfaces between the two, disparate metals become subject to galvanic corrosion. Further, should the coating pattern cover less than all of the stent""s surfaces, the margins between the coating and un-coated regions are subject to galvanic corrosion.
As a further alternative, a stent structure has been described that is formed from a sandwich of structural and radiopaque materials. Three tubes of the materials are codrawn and heat treated to create a structural/radiopaque/structural materials sandwich. Struts and spines (also known as xe2x80x9clinksxe2x80x9d) are then formed in the tube by cutting an appropriate pattern of voids (also known as xe2x80x9ccellsxe2x80x9d) into the tube as is well known in the art. While this approach does provide a stent that is radiopaque and that fulfills the necessary mechanical requirements, the thin cross section of the radiopaque material is nonetheless exposed along the edges of all cut lines. The biocompatibility of the radiopaque material therefore remains an issue and more significantly, a sizeable area may be created that is subject to galvanic corrosion. Any cuts in the sandwich structure cause two disparate metal interfaces, i.e., the juncture between the outer structural layer and the central radiopaque layer as well the juncture between the central radiopaque layer and the inner structural layer, to become exposed along the entire lengths of such cuts.
A stent configuration is therefore required that overcomes the shortcomings inherent in previously known devices. More specifically, a stent structure is needed that provides the requisite mechanical properties for such application, that exposes only fully biocompatible materials to living tissue and that is fluoroscopically visible.
The present invention provides a stent that overcomes the shortcomings of previously known stent devices. The stent fulfills all the mechanical and structural requirements attendant to its function as a stent. Moreover, the stent is fluoroscopically visible without any radiopaque material being exposed to living tissue and without any disparate metal interfaces being subject to galvanic corrosion.
The advantages of the present invention are achieved with the complete encapsulation of radiopaque particles within a binder that is dispersed onto the stent. In one embodiment, a substantially conventional stent is first formed of a structural material by any one of a number of conventional methods. The design should provide sufficient mechanical strength. Radiopaque particles are then placed in a binder which has satisfactory bio- and hemo-compatibility. The binder will then be coated on all surfaces of the stent in such a manner to produce a smooth surface. The thickness and particle loading of the radiopaque material can be adjusted to fine tune the degree of radiopacity needed, depending upon the choice of material used to create the stent. The radiopaque coating/binder may be applied by spraying, dipping, brushing, wiping, pad printing, electrostatic liquid spraying or electrostatic powder coating. Alternatively, therapeutic agents may also be included in the radiopaque coating/binder to serve as a reservoir for controlled drug delivery.
Potential metallic materials for the structural layer of the stent would include, but are not limited to, stainless steels, nickel titanium alloys, cobalt chromium alloys, tantalum and platinum alloys. The thickness in the radial direction of the structural stent should be in the range of about 25-250 microns, preferably in the range of 50-125 microns. Radiopaque materials which are suitable for use are generally materials of high atomic number, located in the bottom two rows coincident with the bottom two rows of the transition metal block of the periodic table. These materials may consist of iodine and its salts, barium and its salts or compounds, tantalum, tungsten, rhenium, osmium, iridium, noble metals, palladium, platinum, gold, colloidal gold, silver and bismuth and its salts or compounds. Oxides and compounds of the metals listed, such as iridium oxide, may also be used. A radiopaque coating/binder thickness should be in the range of about 0.1 to 25 microns, preferably in the range of 1 to 10 microns. Large coating thicknesses may possibly alter the geometry of the stent.
Materials for the binder can be varied and may consist of synthetic polymers or biopolymers. The polymer may be either biostable or bioresorbable. In the case where the polymer is bioresorbable, the radiopaque filler is released. Consequently, a bioresorbable filler such as an iodine salt would be used.
The stent configuration could be used in coronary, carotid, neurological, saphenous vein graph, venous, renal, iliac, biliary, or other peripheral stent designs. The stent may be self-expanding or expandable upon application of an external force, such as the expansion by a dilatation balloon.
These and other features and advantages of the present invention will become apparent from the following detailed description ofpreferred embodiments which, taken in conjunction with the accompanying drawings, illustrate by way of example the principles of the invention.